1. Field of the Invention
The present invention relates, generally, to novel, wet strength resins; to a process for preparing said resins; to a process for treating paper with said resins to improve the off-machine wet strength and dry strength thereof; and to a paper product having improved wet and dry strength properties. More particularly this invention relates to wet strength resins containing maleamic acid and at least one other ethylenically unsaturated monomer, such as vinyl esters, olefins, acrylic and/or methyacrylic acids and/or their esters and/or their amides, and to paper with improved wet and dry strength properties which has been treated with said resins.
2. Description of the Prior Art
In a conventional paper-making operation cellulosic fibers are dispersed in water, drained on a wire screen, pressed into close physical contact and dried. The result is a paper sheet in which the individual fibers are held together by hydrogen bonds which give strength to the dry sheet. When the dry sheet is wet, these hydrogen bonds are broken and the paper loses most of its strength.
To prevent this loss of strength, various chemical treatments have been employed. Among the most successful treatments is the use of synthetic resins which, when added to the cellulosic fibers, either before or after a sheet is formed therefrom, and cured or polymerized, can significantly increase the wet strength of the sheet.
Most commonly used are the urea formaldehyde resins, the product of the condensation of urea and formaldehyde, and melamine-formaldehyde resins, the condensation product of melamine and formaldehyde. An outstanding disadvantage associated with the use of these resins is that if they are not totally cured, a sheet treated with them can emit formaldehyde or toxic aldehydic residues can remain in the sheet. Other resins useful in increasing the wet strength of cellulosic webs include polyamine-epihalohydrin resins, the condensation product of polyamines with epihalohydrins, glyoxalated polyacrylamide resins which are the product of the addition polymerization of N-glyoxalated acrylamide and at least one other ethylenically unsaturated monomer and/or an ethylenically unsaturated basic nitrogen-containing monomer.
Recently, it has become desirable to eliminate the use of formaldehyde-based wet strength resins because of the known and suspected toxic effects of formaldehyde. Formaldehyde-based resins, such as urea-formaldehyde and melamine-formaldehyde resins, cure or crosslink by the formation of intermolecular methyleneamide crosslinkages. Formaldehyde is another product of this crosslinking reaction and can at any time be released during the production or use of products treated with formaldehyde-based resins since the curing or crosslinking reaction continues for an extended period even at room temperature. In order to circumvent the possibility of exposure to formaldehyde, this invention describes wet strength resins which are copolymers of amic acids. These resins function as wet strengthening agents by reaction with the cellulose molecules of a cellulosic web. The reaction takes place between the pendent amide functionalities of the amic acid portion of the copolymers and the hydroxyl groups of the cellulose molecules.
The general reaction between beta-amic acids and alcohols to form half-acid esters was described by Cuculo in a series of publications in the Textile Research Journal, Volume 41 pages 321-326; Volume 41 pages 375-378; volume 43 pages 283-293; volume 45 pages 314-316; volume 46 pages 393-397. These publications describe the preparation of half-acid esters of cellulose by the facile reaction of beta-amic acids and cellulosic hydroxyl groups.
U.S. Pat. No. 3,555,585 describes the preparation of cellulose half-acid esters by reaction of beta-carbamyl or gamma-carbamyl acids with cellulose. The disclosure is concerned with modification of non-woven, cellulosic webs to yield disposable fabrics by the formation of cellulose half-acid esters through the reaction of the cellulosic fabric with a carbamyl substituted organic acid.
The use of salts of copolymers of maleic acid and an ethylenically unsaturated monomer for improving the wet strength of paper has been known since at least 1952. U.S. Pat. No. 2,621,169 granted Dec. 9, 1952 to Robinette et al discloses that ammonium salts of copolymers of styrene and maleic anhydride have utility in improving the wet strength of paper (without specifically teaching how such improvement is obtained). Robinette et al emphasize however that care must be taken in preparing the polymeric compositions of their invention to avoid the formation of amides. Thus the prior art as represented by U.S. Pat. No. 2,621,169 teaches away from the present invention wherein it has been found that resins containing maleamic acid impart wet strength to paper.
U.S. Pat. No. 3,017,291 granted Jan. 16, 1962 to Mclaughlin et al describes the use of emulsion copolymers containing an acid such as maleic or aconitic, and their salts, as wet strength resins. This patent at Column 1, lines 36-39 contains the naked suggestion that salts of partial amides, such as the salt of maleamic acid, can be produced by neutralization of the emulsion copolymers of the patented invention with ammonia and amines. While there is no subsequent enabling disclosure or teaching in respect of such salts merely named by McLaughlin et al, neutralization, as normally understood by one of ordinary skill in the art and in accordance with the disclosure of an antecedent patent assigned to the same assignee, U.S. Pat. No. 2,999,038, at Column 2, lines 50-54, means that aqueous alkali, particularly ammonia, is added to the copolymer. The resulting compound, in the case hypothesized by McLaughlin et al would contain the diammonium salt of maleic acid, not the salt of a half acid, half amide characteristic of a salt of maleamic acid.